Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular CMOS image sensors (“CIS”), has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of the image sensor. Thus, the number of pixels in the pixel array of the image sensor has increased, while the size of each pixel cell has decreased.
Typically each pixel of an image sensor includes a photosensitive element such as a photodiode, and one or more transistors for reading out the signal from the photosensitive element. As pixel cell size decreases, transistors sizes decrease as well. A transfer transistor is commonly used in a pixel with a four-transistor design. The transfer transistor separates the photosensitive element from the rest of the pixel circuitry with the transfer transistor formed between the photosensitive element and a floating node.
In some applications, it is desirable to scale the transfer transistor to have a short gate length in order to achieve greater integration and enhanced pixel fill factor. A short transfer gate length may increase the likelihood of punch-through between the photosensitive element and the floating node. Punch-through occurs when the channel beneath the transfer transistor is depleted and the depletion region surrounding the drain extends through the channel to the source forming a single contiguous depletion region—a detrimental occurrence. On the other hand, a longer transfer gate length may reduce the occurrence of punch-through in conventional pixels, which use an N+ polysilicon transfer gate formed on a flat oxide/silicon substrate. However, a longer transfer gate length may cause problems such as image lag, low sensitivity, and low full well capacity.